Solid-state imagers, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) sensors have commonly been used in photo-imaging applications. A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including a photosensor, for example, a photogate, photoconductor or a photodiode for accumulating photo-generated charge in a specified portion of the imager substrate. Each pixel cell has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some imagers, each pixel cell may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
In a typical CMOS imager, the active elements of the pixel cells perform the functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and amplification of signals representing pixel reset level and pixel image charge. Photo-charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
Examples of CMOS imagers, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imager are described, for example, in U.S. Pat. Nos. 6,140,630; 6,376,868; 6,310,366; 6,326,652; 6,204,524; 6,333,205; and U.S. Pat. No. 6,852,591, all of which are assigned to Micron Technology, Inc. The disclosures of each of the foregoing are hereby incorporated by reference in their entirety.
Microlenses collect light from a large light collecting area and focusing it onto a small photosensitive area of the photosensor. As the size of imager arrays and photosensitive regions of pixel cells continue to decrease, it becomes increasingly difficult to provide a microlens capable of focusing incident light rays onto the photosensitive regions of the pixel cell. This problem is due in part to the increased difficulty in constructing a microlens that has the optimal focal characteristics for the increasingly smaller imager device. Microlens shaping during fabrication is important for optimizing the focal point of a microlens. This in turn increases the quantum efficiency for the underlying pixel cell array.
Conventional microlens fabrication involves an intermediate lens material that is deposited in an array over a substrate and formed into a microlens array using a reflow process. Each microlens is formed with a minimum distance, typically no less than 0.3 microns, between adjacent microlenses. Any closer than 0.3 microns may cause two neighboring microlenses to bridge during reflow. Each microlens is patterned in a material layer as a single square with gaps around it. During reflow of the patterned square microlens material, a gel drop is formed in a partially spherical shape driven by the force equilibrium of surface tension and gravity. The microlenses then harden in this shape. If the gap between two adjacent gel drops is too narrow, they may touch and merge, or bridge, into one larger drop. Bridging changes the shape of the lenses, which leads to a change in focal length, or more precisely the energy distribution in the focal range. A change in the energy distribution in the focal range leads to a loss in quantum efficiency of, and enhanced cross-talk between, pixel cells.
The problem of bridging is exacerbated by recent advances in shared pixel cell architecture. For example, U.S. patent application Ser. No. 11/126,275, assigned to Micron Technology, Inc., the full disclosure of which is hereby incorporated, illustrates two-way and four-way shared pixel cells. Due to the proximity of the photosensors in a shared pixel cell architecture and non-uniform photosensor spacing and/or sizes, the fabrication of microlenses over the photosensors is more prone to bridging.
Accordingly, improved methods of fabricating microlens arrays over pixel cell arrays having uniformly or non-uniformly spaced photosensors and/or uniform or non-uniform photosensor sizes are needed.